Caprolactam can be produced from three hydrocarbon feedstocks: cyclohexane, phenol, and toluene. Approximately 68% of the world""s caprolactam capacity is produced from cyclohexane, 31% from phenol, and 1% from toluene. All of the cyclohexane and phenol-based production proceeds via the formation of cyclohexanone oxime. In 94% of the cyclohexane-based and phenol-based caprolactam capacity, the formation of this oxime requires an ammonia oxidation step.
In the processes involving ammonia oxidation, caprolactam production from cyclohexane or phenol can be broken down into the following steps:
Oxidation of cyclohexane or hydrogenation of phenol, to synthesize cyclohexanone;
Oxidation of ammonia to form nitric oxide, followed by various reaction steps to form a hydroxylamine salt;
Synthesis of cyclohexanone oxime by reaction of cyclohexanone and the hydroxylamine salt; and
Treatment of the cyclohexanone oxime with sulfuric acid followed by neutralization with aqueous ammonia to form caprolactam.
One such method for producing caprolactam is frequently referred to as the xe2x80x9cconventionalxe2x80x9d or xe2x80x9cAllied Signalxe2x80x9d process. Such process is disclosed, for example, in Weissermel and Arp, Industrial Organic Chemistry (VCH Verlagsgesellschaft mbH 1993), pp. 249-258. In the conventional process, hydroxylamine sulfate ((NH2OH)2.H2SO4) and aqueous ammonia are reacted to synthesize the oxime. The hydroxylamine sulfate is produced by the Raschig process:
Catalytic air oxidation of ammonia to form nitric oxide:
4NH3+5O2xe2x86x924NO+6H2Oxe2x80x83xe2x80x83(I)
Continued oxidation of nitric oxide to form nitrogen dioxide:
NO+xc2xdO2xe2x86x92NO2xe2x80x83xe2x80x83(II)
Synthesis of ammonium nitrite:
NO+NO2+(NH4)2CO3xe2x86x922NH4NO2+CO2xe2x80x83xe2x80x83(III)
Reduction of ammonium nitrite to hydroxylamine diammonium sulfate:
2NH4NO2+4SO2+2NH3+2H2Oxe2x86x922HON(SO3NH4)2xe2x80x83xe2x80x83(IV)
Hydrolysis of hydroxylamine diammonium sulfate to hydroxylamine sulfate:
2HON(SO3NH4)2+4H2Oxe2x86x92(NH2OH)2.H2SO4+2(NH4)2SO4+H2SO4xe2x80x83xe2x80x83(V)
Oximating the cyclohexanone with the hydroxylamine sulfate to produce cyclohexanone oxime:
C6H10O+(NH2OH)2.H2SO4+NH4OHxe2x86x92C6H11NO+(NH4)2SO4+H2Oxe2x80x83xe2x80x83(VI)
The process for forming hydroxylamine sulfate in the conventional process is shown in the flow sheet depicted in FIG. 1 of the attached drawing. As shown therein, an air stream 3 is initially compressed in a compressor 10, introduced as a xe2x80x9cprimaryxe2x80x9d air stream through feed line 12 into admixture with a gaseous ammonia stream 1, and thereafter fed to a catalytic ammonia converter 20. Typically, 100% ammonia conversion and 95% selectivity to NO are achieved in that reaction. Upon exiting the converter, some of the NO is further oxidized to NO2 to form an NOx-rich process gas stream 2. The water formed in the ammonia oxidation is thereafter removed from the process stream in a condenser 30. Some of the NO2 is absorbed in the water as it is condensed, producing a weak nitric acid condensate 5.
The NOx-rich process gas stream 7 exiting the condenser 30 is then contacted countercurrently with an aqueous ammonium carbonate stream 9 in a trayed absorption tower 40, referred to as a xe2x80x9cnitrite towerxe2x80x9d. In the conventional process additional, xe2x80x9csecondaryxe2x80x9d air is added either directly into the nitrite tower through line 11 or into the NOx process stream through line 13. The amount of secondary air fed to the nitrite tower affects the relative concentrations of NO and NO2 in the tower. An ammonia stream 15a may also be added to the tower to recover CO2.
Ammonium nitrite is desirably formed in the nitrite tower, according to the reaction:
NO+NO2+(NH4)2CO3xe2x86x922NH4NO2+CO2xe2x80x83xe2x80x83(VII)
The CO2 liberated in this reaction can be recovered in-situ as ammonium carbonate by reaction with the ammonia stream 15, according to the reaction:
CO2+2NH3+H2Oxe2x86x92(NH4)2CO3xe2x80x83xe2x80x83(VIII)
An undesired product, ammonium nitrate, is also formed in the nitrite tower by the following reactions:
2NO2+H2Oxe2x86x92HNO3+HNO2xe2x80x83xe2x80x83(IX)
HNO3+HNO2+2NH3xe2x86x92NH4NO2+NH4NO3xe2x80x83xe2x80x83(X)
Ammonia participating in reaction (X) may be derived from the dissociation of the ammonium compounds formed in these reactions.
The nitrite tower 40 must be operated to minimize the formation of nitrate. To accomplish this, an approximate 1:1 molar ratio of NO to NO2 should be maintained in the tower. In order to maintain such ratio, secondary air is added to the nitrite tower in the conventional process in amounts of about 5 to 10 volume % of the total air flow into the system.
The vent gas 17 exiting the nitriting tower must be properly regulated to minimize the emission of NOx. An increase in production of hydroxylamine sulfate typically results in a corresponding increase in NOx emission in the vent gas 17.
The nitrite-rich aqueous solution 19 is then reacted with a sulfur dioxide stream 21 and an ammonia stream 15b fed into a disulfonate column 50 to form hydroxylamine diammonium sulfate. In some systems the ammonia may rather be admixed with the nitrite-rich aqueous solution 19 from the nitrite tower and the mixture then introduced into the disulfonate column.
The hydroxylamine diammonium sulfate stream 23 removed from the disulfonate column is conventionally hydrolyzed in a hydrolysis column 60 to form hydroxylamine sulfate. A portion of the hydroxylamine diammonium sulfate is recycled through line 27 to the disulfonate column 50. The hydroxylamine sulfate solution exiting the hydrolysis column is then recovered from line 25 for use in the oximation process.
In view of the strict environmental regulation of NOx emissions, the quantity of NOx gases vented through line 17 cannot be increased. Accordingly, any increased hydroxylammonium sulfate production (and subsequent caprolactam production) must be obtained without any increase in NOx emissions. This can be accomplished by increasing the amount of air and ammonia fed to the process while increasing the plant size, e.g., the size of the nitrite tower 40 and air compressor 10. However, such an increase in equipment capacity requires substantial capital investment.
There is therefore a need for the development of improved techniques in the conventional process for producing caprolactam, by which increased amounts of hydroxylamine sulfate and, consequently, caprolactam can be produced without large capital investment, and without increasing NOx emissions.
The present invention provides just such an improvement in the conventional process for the production of caprolactam involving:
(a) reacting air with ammonia gas in an ammonia conversion zone to produce nitric oxide;
(b) oxidizing a portion of the nitric oxide to nitrogen dioxide to produce an NOx-rich process gas stream;
(c) reacting the NOx-rich stream with ammonium carbonate in a nitriting zone to produce ammonium nitrite;
(d) reducing the ammonium nitrite to hydroxylamine diammonium sulfate;
(e) hydrolyzing the hydroxylamine diammonium sulfate to hydroxylamine sulfate;
(f) oximating the hydroxylamine sulfate with cyclohexanone to produce cyclohexanone oxime; and
(g) converting the cyclohexanone oxime to caprolactam.
In accordance with the invention, the foregoing process is improved by adding supplemental oxygen downstream of the ammonia conversion zone to increase the quantity and rate of formation of nitrogen dioxide in the NOx-rich process gas stream. Desirably, secondary air, normally introduced into the nitriting zone (or into the NOx-rich gaseous stream feeding into the nitriting zone) is rerouted to the ammonia conversion zone to increase the production of nitric oxide formed in the ammonia conversion zone without increasing the level of NOx contained in the gas vented from the nitriting zone.
Utilizing the improved technique of the invention, desirably by rerouting the secondary air to the ammonia conversion zone and maintaining the volumetric percentage of ammonia fed to the conversion zone at a constant or increased level, the production of NO in the conversion zone is increased. By adding supplemental oxygen according to the invention, both the amount and rate of conversion of NO to NO2 are increased, thereby promoting formation of nitrite in the nitriting zone, without any adverse effect on the NOx content of gases vented from the nitriting zone. Alternatively, the addition of supplemental oxygen may be used to lower NOx emissions, with or without rerouting of secondary air to the ammonia conversion zone, and with or without increases in nitrite (and consequently hydroxylamine sulfate and caprolactam) production. The invention also encompasses adding supplemental oxygen according to the invention without rerouting secondary air to the ammonia converter, but increasing the volumetric percentage of ammonia fed to the conversion zone to increase production of NO. This ultimately results in an increase in formation of hydroxylamine sulfate and caprolactam without an increase in NOx emissions.
The method of the present invention thus facilitates an increase in hydroxylamine sulfate production in the conventional process for synthesizing caprolactam, while maintaining NOx emissions at constant, or decreased, levels. It is estimated that use of the method of the invention normally results in an increase of between about 5 and 15% in the production of hydroxylamine sulfate without increasing NOx emissions. Furthermore, this is accomplished without substantial capital investment, such as would otherwise be required to increase plant capacity. Moreover, by substituting oxygen for inert nitrogen present in the secondary air conventionally fed to the nitriting zone, the oxygen partial pressure in the system may be increased and residence times for the intermediates formed in the various stages of the process may be lowered.
In the production of nitric acid, it is known that direct injection of supplemental oxygen can boost nitric acid synthesis while controlling NOx emissions. Such addition of oxygen is described, for example, in U.S. Pat. Nos. 4,183,906; 4,183,906; 4,235,858; and 5,167,935; UK Patent No. 803211; and EP published Patent Applications Nos. 799794 and 808797. Oxygen addition is also described in Kongshaug, Extension of Nitric Acid Plant Capacity by Use of Oxygen, Nitric Acid Symposium (1981); and by Faried et al., Boosting Existing Nitric Acid Production, The Fertiliser Society (1986). For example, EP 808797 describes an improved process for nitric acid production in which supplemental oxygen is added to the cooler/condenser, the absorption tower, the ammonia converter, and/or the bleacher, to cause an increase in nitric acid production without increasing NOx emissions. No supplemental oxygen addition of this type is believed to have been previously disclosed in connection with the synthesis of caprolactam.
Feeding oxygen to the ammonia converter has been employed in the BASF and Inventa processes for the synthesis of caprolactam. (As described, for example, in the Kirk Othmer Encyclopedia of Chemical Technology, 4th edition, 4:831 (1992) and U.S. Pat. No. 5,777,163.) In these processes, however, no supplemental oxygen is added downstream of the converter. Also, the BASF and Inventa processes differ substantially from the conventional process for producing caprolactam in that they do not add air to the ammonia converter, and do not involve the formation of NO2.